Hurricane Season Terminology

Eye: The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.

Eyewall / Wall Cloud: An organized band or ring of cumulonimbus clouds that surround the eye, or light-wind center of a tropical cyclone. Eyewall and wall cloud are used synonymously.

Extratropical: A term used to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of energy from evaporating ocean water to the temperature contrast between warm and cold air masses. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.

Fujiwhara Effect: The tendency of two nearby tropical cyclones to rotate cyclonically about each other.

Gale Warning: A warning of 1-minute sustained surface winds in the range 39 mph (34 kts.) to 54 mph (47 kts.) inclusive, either predicted or occurring and not directly associated with tropical cyclones.

High Wind Warning: A high wind warning is defined as 1-minute average surface winds of 40 mph (35 kt) or greater lasting for 1 hour or longer, or winds gusting to 58 mph (50 kts.) or greater regardless of duration that are either expected or observed over land.

Hurricane: A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 74 mph (64 kts.) or more. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian.

Hurricane Season: The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30.

Hurricane Warning: A warning that sustained winds 74 mph (64 kts.) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

Hurricane Watch: An announcement for specific coastal areas that hurricane conditions are possible within 36 hours.

Landfall: The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone's strongest winds to be experienced over land even if landfall does not occur. Similarly, it is possible for a tropical cyclone to make landfall and have its strongest winds remain over the water.

Storm Surge: An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.

Storm Tide: The actual level of sea water resulting from the astronomic tide combined with the storm surge.

Storm Warning: A warning of 1-minute sustained surface winds of 55 mph (48 kts.) or greater, either predicted or occurring, not directly associated with tropical cyclones.

Subtropical Cyclone: A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection. A second type of subtropical cyclone is a small-scale low pressure center originating in or near a developing frontal zone of horizontal wind shear, with radius of maximum sustained winds generally less than 30 miles. The entire circulation may initially have a diameter of less than 100 miles. These generally short-lived systems may be either cold core or warm core.

Subtropical Depression: A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 38 mph (33 kts.) or less.

Subtropical Storm: A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 39 mph (34 kts.) or more.

Tropical Cyclone: A warm-core (warm at the surface and several miles above the ground) non-frontal cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere.

Tropical Depression: A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 38 mph (33 kts.) or less.

Tropical Disturbance: A discrete tropical weather system of apparently organized convection -- generally 100 to 300 nautical miles in diameter -- originating in the tropics or subtropics and maintaining its identity for 24 hours or more.

Tropical Storm Warning: A warning that sustained winds within the range of 39 to 73 mph (34 to 63 kts.) associated with a tropical cyclone are expected in a specified coastal area within 24 hours or less.